1. Field of the Invention
This application relates to wire insulation. More particularly, this application relates to profiled insulation for LAN cables.
2. Description of Related Art
Copper cables are used for a variety of tasks, such as power transmission and signal transmission. In signal transmission tasks, the choice of insulation is of particular concern. For example, twisted pairs of copper conductors used in data cables (e.g. LAN (Local Area Network) cables) must meet certain fire safety standards and be cost effective, while minimizing signal degradation. Such signal degradation may be caused by factors such as interference with adjacent conductors, and inductance ith the insulation.
Thus, in developing copper wire signal cables, often having multiple twisted pairs of copper wire within the same jacket, there are the competing concerns of minimizing cost while maximizing signal strength and clarity. FIG. 1 shows a common prior art design having eight conductors grouped into four twisted pairs, in this example shown with an optional cross filler. In order for the cable to function properly, the impedance measurement between the two copper conductors of a twisted pair must be precisely maintained. This is achieved by insulating the conductor with a dielectric material. However, the dielectric material has a negative impact on the electrical signal and contributes to signal losses as well as other undesirable electrical phenomena. In addition, this dielectric material adds cost to the cable construction and often has a negative impact on cable fire performance, such as in UL™ (Underwriters Laboratories) testing. Thus, it is desirable to find ways to reduce the amount of dielectric material in proximity to the copper conductor without affecting the impedance (e.g. target of 100 ohms) between the two copper conductors forming the twisted pair.
Several approaches have been taken in the past to reduce the amount of dielectric material in proximity to the copper conductors without reducing the impedance of the twisted pair made from said copper conductors. For example, some manufacturers have replaced typical copper wire dielectric insulation with a foamed dielectric insulation which adds a gas component to the insulation. This yields a reduction in the amount of dielectric material necessary to maintain the impedance of the twisted pair. It is known that the typical gases used to foam dielectric materials have a dielectric constant close to 1 (most desirable), whereas known dielectric materials without the gas component have a dielectric constant substantially greater than 1, so this approach would appear, at first glance, to aid in resolving the concerns. However, this method not only increases the complexity of the extrusion process, but often requires additional manufacturing equipment. It is also difficult to manufacture a data communications cable with good electrical properties using this type of process.
Another method to reduce the amount of insulation while simultaneously maintaining the impedance between a twisted pair of conductors is to add openings (air or inert gas filled) within the insulation itself. However, prior art methods for producing such insulation with longitudinal air/gas openings require complex extrusion designs that may not produce the intended results or have otherwise produced ineffective results due to failure to maintain stable production of the openings during manufacturing.
Yet another manner for maintaining the impedance between a twisted pair of conductors while reducing the amount of insulation material used within a signal cable is to use what is termed “profiled” insulation. Profiled insulation refers to an insulation that is provided around a copper wire conductor, the cross-section of which is other than substantially circular. Such examples of profiled insulation may include saw tooth structures or other similar designs intended to both separate the conductors from one another while using less insulation than a solid insulator of similar diameter but yielding the same impedance between twisted pairs of conductors. One Example, of this type of insulation may be found in U.S. Pat. No. 7,560,646. See prior art FIG. 2.
In this arrangement, peak to peak contact between the profiled insulation of two conductors in a pair is desirable so as to maximize the distance between the conductors. This is shown for example in FIG. 3. However, owning to inconsistencies in the twinning process (where the two conductors are twisted around one another to form the twisted pair) at some points, the peak of one conductor insulation may “nest” into a valley of an adjacent conductor insulation as shown in FIG. 4. This situation undesirably shortens the distance between the conductors negatively affecting impedance. Moreover, if the nesting occurs periodically, the result is that along the pair at some points there is peak to peak contact and at other points there is peak to valley contact resulting in inconsistent impedance measurements along the length of the pair.
It is noted that certain prior art documents such as U.S. Patent Publication No. 2009/0229852 teaches the forward and/or back twisting (explained in more detail below) of profiled insulation for ensuring nesting. With profiled insulations, the peaks and valleys run longitudinally. The twinning operation of two conductors around one another inherently imparts some twist to the profiled insulation on each conductor. This prior art arrangement uses a back-twisting operation to counter this inherent twisting of the profiled insulation so that the peaks and valleys in the pair remain longitudinal to that corresponding peaks and valleys on the insulations of the two conductors in the pair match and thus more easily nest. As noted in the penultimate paragraph of the '852 application, the resulting impedance measurements are improved because in peak to peak contact designs, the peaks may crush during the twinning process